"The officer's equipment belt including his firearm had been secured in the vehicle," Smyth said Monday. "Police believe the two accused unlawfully entered the vehicle and stole several items including the firearm from the off-duty officer."

Just after midnight later that night, the two suspects exchanged words with a group of young people sitting in a vehicle in a convenience store parking lot. Smyth said one of the suspects allegedly approached the car with a gun and took aim at those inside.

A 16-year-old girl was shot once in the upper body and rushed to hospital in critical condition. The shooting was "senseless and reckless" without apparent motive, Smyth said.

"It's not believed that she was the intended target of the attack," he said. "She sustained critical injuries. Her parents are with her and members of the police service send our sincerest hopes for a complete recovery."

When officers tried to arrest one of the suspects on Sunday, Smyth said an armed standoff ensued but ended peacefully. The other suspect was arrested without incident.

Investigators are still performing ballistics and forensic tests to confirm if the gun used was stolen from the RCMP, Smyth said. He wouldn't say whether RCMP officers are allowed to leave their guns inside unattended vehicles or how the gun was "secured" inside the car.

"I can only speak for the Winnipeg police service," Smyth said. "We don't have a provision for our officers to store their weapons in unattended vehicles."

The RCMP declined to answer any questions about the alleged stolen gun or about whether officers can leave their guns unattended in vehicles.

Spokesman Robert Cyrenne issued a written statement saying Winnipeg police are leading the investigation.

"The RCMP thanks the Winnipeg Police Service for their diligent efforts in quickly tracking and securing the firearm that was stolen from a marked RCMP police vehicle," the statement read. "As the Winnipeg Police Service investigation into the matter is ongoing, no further information can be provided at this time."